Lithography in semiconductor processing relates generally to the process of transferring patterns which correspond to desired circuit components onto one or more thin films which overlie a substrate. One important step within the field of lithography involves optical tools and methods for transferring the patterns to the films which overlie the semiconductor wafer. Patterns are transferred to a film by imaging various circuit patterns onto a photoresist layer which overlies the film on the wafer. This imaging process is often referred to as "exposing" the photoresist layer. The benefit of the exposure process and subsequent processing allows for the generation of the desired patterns onto the film on the semiconductor wafer, as illustrated in prior art FIG. 1a-1f.
Prior art FIG. 1a illustrates a photoresist layer 10 deposited by, for example, spin-coating, on a thin film 11 such as, for example, silicon dioxide (SiO.sub.2) which overlies a substrate 12 such as silicon. The photoresist layer 10 is then selectively exposed to radiation 13 (e.g., ultraviolet (UV) light) via a photomask 14 (hereinafter referred to as a "mask") to generate one or more exposed regions 16 in the photoresist layer 10, as illustrated in prior art FIG. 1b. Depending on the type of photoresist material utilized for the photoresist layer 10, the exposed regions 16 become soluble or insoluble in a specific solvent which is subsequently applied across the wafer (this solvent is often referred to as a developer).
The exposed regions 16 are made either soluble or insoluble in the developer. When the exposed regions 16 are made soluble, a positive image of the mask 14 is produced in the photoresist layer 10, as illustrated in prior art FIG. 1c, and the photoresist material is therefore referred to as a "positive photoresist". The exposed underlying areas 18 in the film 11 may then be subjected to further processing (e.g., etching) to thereby transfer the desired pattern from the mask 14 to the film 11, as illustrated in prior art FIG. 1d (wherein the photoresist layer 10 has been removed). Conversely, when the exposed regions 16 are mode insoluble, a negative image of the mask 14 is produced in the photoresist 10 layer, as illustrated in prior art FIG. 1e, and the photoresist material is therefore referred to as a "negative photoresist". In a similar manner, the exposed underlying areas 20 in the film 11 may then be subjected to further processing (e.g., etching) to thereby transfer the desired pattern from the mask 14 to the film 11, as illustrated in prior art FIG. 1f.
The transfer of patterns to the photoresist layer 10 as discussed above involves the use of optical aligners. Optical aligners are machines which contain a variety of subsystems that work together to form the imaging function. Such optical aligners include: (1) an illumination source which provides the optical energy (UV light in the above example) for transforming the photoresist via exposure, (2) an optical subsystem that focuses the circuit patterns onto the photoresist surface and allows for controlled exposure times, and (3) and a movable stage that holds the wafer being exposed.
Historically, three primary methods have been used to optically transfer a mask pattern to a photoresist covered film. These methods are: contact printing, proximity printing and projection printing and are illustrated in simplified form in prior art FIGS. 2a-2d, respectively. Contact printing 100, as illustrated in prior art FIG. 2a, was the earliest method used to produce patterns. Contact printing 100 involves a light source 112, an optical system 114, a mask 116 and a photoresist layer 118 overlying a thin film 119 (not shown) which, in turn, overlies a semiconductor wafer 120. The mask 116, which contains the desired circuit patterns for transfer to the photoresist layer 118, is positioned (aligned) relative to any existing patterns that already existed on the wafer 120. The mask 116 is then clamped down to the photoresist layer 118, thereby making physical contact with the photoresist layer 118, and exposed with ultraviolet (UV) light from the light source 112. This method provides for an excellent image transfer and good resolution (i.e., good minimum linewidth spacing).
Contact printing, however, suffers from the direct contact made between the mask 116 and the photoresist layer 118. The repeated contact made between the mask 116 and the photoresist layer 118 in the process results in defects generated in the mask 116 which are then reflected in the transfer made on subsequently processed wafers. To prevent this problem, the masks 116 must be disadvantageously inspected and cleaned regularly. In addition, small particles may be caught between the mask 116 and the photoresist layer 118 when affixing the two elements, thereby preventing the desired direct contact between the mask 116 and the photoresist layer 118. This particulate contamination results in reduced resolution in the area local to the foreign particle. Consequently, contact printing is not common in VLSI semiconductor manufacturing.
Proximity printing 122, as illustrated in prior art FIG. 2b, involves placing the mask 116 near the wafer 120 (which is covered with the photoresist 118) during exposure, however, the mask 116 and the wafer 120 do not make contact. By introducing a gap 124 between the mask 116 and the wafer 120, the defect problem of contact printing is substantially avoided. Unfortunately, as the gap 124 increases, the resolution of the proximity printing system 122 rapidly deteriorates. For example, a 10 .mu.m gap with a 400 nm exposure (the wavelength of the light source 112) results in a minimum resolution of about 3 .mu.m. In addition, proximity printing 122 requires extremely flat masks 116 and wafers 120 in order to prevent gap variations spatially about the wafer 120. Since many VLSI semiconductor circuits today require features of 0.25 .mu.m or less, proximity printing 122 is not considered adequate for many VLSI semiconductor manufacturing operations.
Projection printing is a generic term that encompasses various pattern transfer techniques. These techniques, for example, include: (a) projection scanning systems, (b) reduction (e.g., 4X or 10X) step-and-repeat projection systems and (c) reduction step-and-scan systems. In each system, lens elements or mirrors are used to focus the mask image on the wafer surface (containing the photoresist).
Projection scanning systems (often called scanning projection aligners), use a reflective spherical mirror (reflective optics) to project an image onto the wafer surface, as illustrated, for example, in prior art FIG. 2c. The system 126 includes a primary mirror 128 and a secondary mirror 129 which are arranged with the mask 116 and the wafer 120 to image the mask pattern onto the photoresist layer 118 which overlies the film 119 on the wafer 120 (the photoresist layer 118 and the thin film 119 are not shown in FIG. 2c for simplicity). A narrow arc of radiation is imaged from the mask 116 to the wafer 120 with light that travels an optical path that reflects the light multiple times. The mask 116 and the wafer 120 are scanned through the arc of radiation by means of a continuous scanning mechanism (not shown). The scanning technique minimizes mirror distortions and aberrations by keeping the imaging illumination in the "sweet spot" of the imaging system 128 and 129.
Reduction step-and-repeat systems 130 (also called reduction steppers) use refractive optics (as opposed to reflective optics in the system 126 of prior art FIG. 2c) to project the mask image onto the photoresist layer 118 which overlies the firm 119 on the wafer 120, as illustrated, for example, in prior art FIG. 2d. The reduction stepper 130 includes a mirror 132, a light source 134, a filter 136, a condenser lens system 138, a reticle 140, a reduction lens system 142 and the wafer 120. The mirror 132 behaves as a collecting optics system to direct as much of the light from the light source 134 (e.g., a mercury-vapor lamp) to the wafer 120. The filter 136 is used to limit the light exposure wavelengths to the specified frequencies and bandwidth. The condenser system 138 focuses the radiation through the reticle 140 and to the reduction lens system to thereby focus a "masked" radiation exposure onto a limited portion of the wafer 120, namely onto a single semiconductor die 144.
Since it is complex and expensive to produce a lens capable of projecting a mask pattern of an entire 150 mm or 200 mm wafer, the refractive system 130, as illustrated in prior art FIG. 2d, projects an image only onto a portion of the wafer 120 corresponding to an individual semiconductor die 144. This image is then stepped and repeated across the wafer 120 in order to transfer the pattern to the entire wafer (and thus the name "steppers"). Consequently, the size of the wafer is no longer a consideration for the system optics.
The reduction stepper system 130 thus uses the reticle 140 instead of a mask. Reticles are similar to masks, but differ in that a mask contains a pattern for transfer to the entire wafer in one exposure while a reticle contains a pattern image for a single or several semiconductor die that must be stepped and repeated across the wafer 120 in order to expose the entire wafer substrate. Current reduction stepper systems such as the system 130 utilize reticles that contain a pattern that is an enlargement of the desired image on the wafer 120. Consequently, the reticle pattern is reduced when projected onto the wafer 120 during exposure (and thus the name "reduction stepper").
One advantage of stepper technology over the full wafer scanning type technology is higher image resolution (i.e., smaller minimum linewidths). In addition, stepping each die on the wafer 20 allows compensation for wafer distortion. Further still, reduction steppers provide good overlay accuracy. Steppers do, however, exhibit reduced throughput (number of wafers/hour) and require precision control of the mechanical stage (not shown) which holds the wafer 120. The advantages of reduction steppers, however, presently outweigh their disadvantages and thereby make reduction steppers quite popular in the manufacture of VLSI semiconductors with minimum linewidths less than 1 .mu.m.